FIG. 1 illustrates the general construction of color cathode ray tubes to which this invention relates. The tube's glass envelope comprises a substantially rectangular panel 1 connected to a neck 3 by a funnel 2. On the inner surface of the panel 1 is a phosphor screen with a plurality of vertically-aligned phosphor stripes which emit red, green, or blue light when struck by electron beams. The source of the electron beams is an in-line electron gun 6 in the neck, emitting three electron beams 10 in a generally horizontal direction, the three beams corresponding to the three colors of phoshor stripes. Between the electron guns and the phosphor screen, and closely adjacent to the phosphor screen, is shadow mask 5 having a number of apertures 52 in its major face 51. Shadow mask 5 also has a skirt portion 8 extending rearward from the periphery of major face 51. Skirt 8 is held in place by a mask frame 7 of L-shaped cross section. Frame 7, in turn, is attached through springs 9 to pins (not shown) buried in the inner side walls of panel 1.
As the electron beams travel from the electron guns to the phosphor screen, they are deflected by a deflection yoke (not shown) surrounding the outside of funnel 2 so as to scan the entire area of screen 4 in the familiar raster pattern. The function of the shadow mask is to permit each electron beam to strike the correct color phosphor stripes while at the same time preventing it from striking any other color phosphor stripe. The electron beams therefore pass through the mask's apertures but are absorbed where they strike the mask at a point in which no aperture is formed.
Only about one-third of the total electron beam energy leaving the electron guns reaches the phosphor screen. The rest is intercepted by the shadow mask and converted into heat, so that the normal operating temperature of the shadow mask generally reaches 353 K. Since the shadow mask is generally made of a thin plate (0.1-0.3 mm) of cold rolled steel which essentially consists of iron, it has a large thermal expansion coefficient, as much as about 1.2.times.10.sup.-5 /K (at a temperature of 273 K.-373 K.). The mask frame, which supports the skirt portion of the shadow mask, is much heavier, being made of cold rolled steel of about 1.0 mm thickness. The mask frame also is generally coated with a black oxide layer. Consequently, when the shadow mask is bombarded with electron beams, heat will readily be conducted or radiated from the skirt portion of the mask to the mask frame, and the temperature of the periphery of the major face of the shadow mask will be considerably lower than the temperature of the center of the major face. This temperature differential causes a distortion in the mask called "doming." Doming is a localized distortion, caused by differential heating, in which a portion (commonly the central portion) of the shadow mask's major face expands more than another portion (commonly the periphery) of the major face, causing the warmer portion to swell in the direction of the screen. As a result, the distance between the shadow mask and the phosphor screen is reduced below the proper distance necessary to maintain the critical alignment between each electron beam, the apertures, and the corresponding phosphor stripes. Color purity is therefore deteriorated. Doming is particularly noticeable when the device (television receiver or monitor) containing the color cathode ray tube is first turned on. It is also particularly noticeable when one portion of the image being produced by the color cathode ray tube is much brighter than the rest of the image.
Various schemes have been proposed to solve the heating problem. For example, U.S. Pat. No. 2,826,583 issued on Mar. 11, 1958, showed a black carbon layer deposited on the surface of the shadow mask to improve radiation. However, because of the constant expansion and contraction of the shadow mask, pieces of these black carbon layers break off inside the envelope and create problems by lodging in apertures of the shadow mask or in various locations within the electron gun.
In U.S. Pat. No. 3,887,828, issued on June 3, 1975 (and the corresponding Japanese Patent No. Disclosure 50-44771, disclosed on Apr. 22, 1975), it is proposed that the electron gun side of the shadow mask be covered in succession by three layers of material: a porous manganese dioxide layer, an aluminum layer, and a nickel oxide or nickel-iron oxide evaporated layer. In this type of shadow mask, heat generated by electron beam bombardment spreads throughout the surface of the triple layer but is not conducted to the shadow mask because the thermal conductivity of porous manganese dioxide is extremely low. This triple-layered shadow mask effectively prevents the high temperatures which cause shadow mask doming; but the article is ill suited for mass production because of the large scale of equipment and long production time necessary to produce such a mask.